1. Field of the Invention
The present invention relates to a micro-resonator, a frequency filter including the micro-resonator and a communication apparatus provided with a filter based on the micro-resonator.
2. Description of the Related Art
A demand for wireless network technology has been rising due to a reason that the number of devices that use a network increases remarkably in recent years with progress in information technology and in light of usability. In an RF (high frequency) front-end module used in wireless communication, there exist components of a comparatively large size such as a RF (high frequency) filter, a SAW (surface acoustic wave) filter and dielectric filter used as an IF (intermediate frequency) filter as well as a semiconductor chip, and the existence thereof has prevented miniaturization and cost reduction of the RF front-end. It is desired lately to incorporate those filtering functions into a semiconductor chip.
On the other hand, attention has been paid in recent years to a micro-machine (MEMS: Micro Electro Mechanical Systems) element and a small device in which a MEMS element is incorporated. A basic characteristic of the MEMS element is that a driving body constructed as a mechanical structure is incorporated into a part of the element, and the driving body is electrically driven by an application of coulomb force between electrodes and the like.
Since a micro-oscillation element formed by using a micro-machining technology based on a semiconductor process has such advantages that an area occupied by a device is small, a high Q-value can be obtained and an integration with another semiconductor device is possible, a use as an intermediate frequency (IF) filter and high frequency (RF) filter in wireless communication devices has been proposed by research laboratories including Michigan University (refer to Non-patent Reference 1).
FIG. 1 is a schematic view of an oscillator, specifically a MEMS-type resonator, constituting a high frequency filter described in Non-patent reference 1. This resonator 1 includes an input-side wiring layer 4 and an output electrode 5 made of polycrystalline silicon, for example, formed on a semiconductor substrate 2 through an insulation film 3 and a beam made of, for example, polycrystalline silicon capable of oscillating, that is, what is called a beam-type oscillation electrode 7 is formed to face the output electrode 5 separated by a space 6. The oscillation electrode 7 is connected to the input-side wiring layer 4 by straddling the output electrode 5 like a bridge such that anchor portions (support portions) 8 [8A, 8B] on both ends support this electrode. The oscillation electrode 7 becomes an input electrode. An input terminal t1 is led out from a gold (Au) film 9 of the input-side wiring 4, and an output terminal t2 is led out from the output electrode 5.
In this resonator 1, a high frequency signal S1 is supplied to the oscillation electrode 7 through the input terminal t1 in a state where a DC bias voltage V1 is applied between the oscillation electrode 7 and the ground. Specifically, when the DC bias voltage V1 and high frequency signal S1 are input from the input terminal t1, the oscillation electrode 7 that has a natural oscillation frequency determined by a length L oscillates by electrostatic generated between the output electrode 5 and the oscillation electrode 7. By means of the oscillation, a high frequency signal corresponding to a temporal change of capacitance between the output electrode 5 and the oscillation electrode 7 and the DC bias voltage is output from the output electrode 5 (therefore, from the output terminal t2). In the high frequency filter, a signal corresponding to the natural oscillation frequency (resonant frequency) of the oscillation electrode 7 is output.
[Non-patent Reference 1] IEEE Journal of Solid-state Circuits, Vol. 35, No. 4, April 2000